1. Field of the Invention
The present invention relates to liquid cleansing compositions of the type which are typically used as skin cleansing or shower gel compositions. In particular, the invention relates to lamellar phase compositions which are readily shear thinning (i.e., can suspend large particles, such as 1 micron and larger, because of their high zero shear viscosity; and yet still readily pour). In addition, the product also "heaps" on dispensing and is soft and lotion-like, thereby providing consumers a signal of enhanced moisturization.
2. Background of the Invention
The rheological behavior of all surfactant solutions, including liquid cleansing solutions, is strongly dependent on the microstructure, i.e., the shape and concentration of micelles or other self-assembled structures in solution.
When there is sufficient surfactant to form micelles (concentrations above the critical micelle concentration or CMC), for example, spherical, cylindrical (rod-like) or discoidal micelles may form. As surfactant concentration increases, ordered liquid crystalline phases such as lamellar phase, hexagonal phase or cubic phase may form. The lamellar phase, for example, consists of alternating surfactant bilayers and water layers. These layers are not generally flat but fold to form submicron spherical onion like structures called vesicles or liposomes. The hexagonal phase, on the other hand, consists of long cylindrical micelles arranged in a hexagonal lattice. In general, the microstructure of most personal care products consist of either spherical micelles; rod micelles; or a lamellar dispersion.
As noted above, micelles may be spherical or rod-like. Formulations having spherical micelles tend to have a low viscosity and exhibit newtonian shear behavior (i.e., viscosity stays constant as a function of shear rate; thus, if easy pouring of product is desired, the solution is less viscous and, as a consequence, it doesn't suspend as well). In these systems, the viscosity increases linearly with surfactant concentration.
Rod micellar solutions are more viscous because movement of the longer micelles is restricted. At a critical shear rate, the micelles align and the solution becomes shear thinning. Addition of salts increases the size of the rod micelles thereof increasing zero shear viscosity (i.e., viscosity when sitting in bottle) which helps suspend particles but also increases critical shear rate (point at which product becomes shear thinning; higher critical shear rates means product is more difficult to pour).
Lamellar dispersions differ from both spherical and rod-like micelles because they can have high zero shear viscosity (because of the close packed arrangement of constituent lamellar droplets), yet these solutions are very shear thinning (readily dispense on pouring). That is, the solutions can become thinner than rod micellar solutions at moderate shear rates.
In formulating liquid cleansing compositions, therefore, there is the choice of using rod-micellar solutions (whose zero shear viscosity, e.g., suspending ability, is not very good and/or are not very shear thinning); or lamellar dispersions (with higher zero shear viscosity, e.g. better suspending, and yet are very shear thinning).
To form such lamellar compositions, however, some compromises have to be made. First, generally higher amounts of surfactant are required to form the lamellar phase. Thus, it is often needed to add auxiliary surfactants and/or salts which are neither desirable nor needed. Second, only certain surfactants will form this phase and, therefore, the choice of surfactants is restricted.
In short, lamellar compositions are generally more desirable (especially for suspending emollient and for providing consumer aesthetics), but more expensive in that they generally require more surfactant and are more restricted in the range of surfactants that can be used.
When rod-micellar solutions are used, they also often require the use of external structurants to enhance viscosity and to suspend particles (again, because they have lower zero shear viscosity than lamellar phase solutions). For this, carbomers and clays are often used. At higher shear rates (as in product dispensing, application of product to body, or rubbing with hands), since the rod-micellar solutions are less shear thinning, the viscosity of the solution stays high and the product can be stringy and thick. Lamellar dispersion based products, having higher zero shear viscosity, can more readily suspend emollients and are typically more creamy. Again, however, they are generally more expensive to make (e.g., they are restricted as to which surfactants can be used and often require greater concentration of surfactants).
Unexpectedly, applicants have now found that if certain liquid fatty acids (e.g., long chain, unsaturated and/or branched fatty acids); long chain, unsaturated and/or branched alcohols (e.g., oleyl alcohol or isostearyl alcohol) or derivatives (ester of fatty acids and ether of fatty alcohols) of these fatty acids and/or alcohols are used in a typical rod-micellar solution, a lamellar phase can be induced.
Specifically, applicants have found that, in compositions comprising (1) one or more anionic surfactants; (2) at least one amphoteric and/or zwitterionic surfactant; and (3) optionally, one or more nonionic surfactants; when a liquid structurant as noted above (i.e., long chain, unsaturated and/or branched liquid fatty acid; long chain unsaturated and/or branched liquid alcohols or derivatives thereof having a melting point below about 25.degree. C. is added, the compositions become lamellar phase compositions. Further, with these specific structurants, there is no crystallization of the structurant.
The use of fatty acids generally in liquid cleansers (shower gels and shampoos) is taught, for example, in WO 94/17166 to Giret et al. (assigned to Procter & Gamble); WO 94/18737 to Cothran et al. (assigned to Procter & Gamble) and in U.S. Pat. Nos. 5,132,037; 5,234,619; and 5,290,470; each to Greene et al.
Each of these references, however, teach the use of linear, saturated fatty acids (versus the unsaturated or branched fatty acids of the subject invention) which are insoluble and which crystallize in the products. Indeed, in these references, it is intended to crystallize the fatty acids since this is an important factor in the structuring, (see WO 93/18737 at page 5, lines 23-32). These references also do not teach unsaturated or branched, long chain alcohols or their ether derivatives.
Dias et al. (WO 94/01084, U.S. Pat. No. 5,308,526), MacGilp et al. (U.S. Pat. Nos. 5,158,699; 5,296,157; WO 92/15666) and Torres (WO 94/01085) teach the use of free fatty acids and potassium fatty acid soap where said fatty acid has an Iodine Value between 0 to 15 (i.e., iodine values indicate level of saturation of the fatty acids). In addition, there is no teaching of unsaturated or branched, long chain fatty acid or alcohol.
U.S. Pat. No. 5,360,581 to Rizvi et al. teach the use of a long chained saturated fatty acid (preferably C.sub.22) with polyethyleneimine to increase stability of the product. The fatty acid is saturated compared to the liquid, unsaturated fatty acid of the subject invention.
Japanese patent JP 7,025,726 teaches the use of liquid fatty acids in emulsion. The patent teaches compositions having 30% or greater oil by which is meant an emollient (e.g., vegetable oil) while the compositions of the subject invention comprise no more than about 20%, preferably no more than about 15% by wt. oil/emollient. Furthermore, JP 7,025,726 does not teach the use of liquid fatty acid as a structurant while the present invention utilizes the liquid fatty acid to generate the lamellar phase and thus structures the product.
Applicants' copending application Ser. No. 08/469,949 to Shana'a, now abandoned, relates to soap composition which comprises 5 to 35% C.sub.8 to C.sub.22 fatty acid of which 20-50% must be un-neutralized (i.e., more than 50% is neutralized to form soap). The present application is a soap-free application and such compositions are generally milder. Moreover, it would not have been obvious that addition of the structurants of the subject invention in the absence of soap would yield lamellar phase compositions.
U.S. Pat. No. 5,612,307 to Chambers et al. teaches aqueous liquid and moisturizing compositions comprising surfactant active agent and benefit agent stripes which are separate, but combinedly dispensable. In the benefit stripe, oleic acid or isostearic may be used. However, these are among a long, comprehensive list of "possible" emollients. There is no teaching or suggestion that specific compounds, among dozens and dozens, have an unexpected lamellar structuring effect.